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Evolution Basics: Becoming Human, Part 1: Mitochondrial Eve and Y Chromosome Adam

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March 13, 2014 Tags: Adam, the Fall, and Sin, Genetics, History of Life, Human Origins

Today's entry was written by Dennis Venema. You can read more about what we believe here.

Evolution Basics: Becoming Human, Part 1: Mitochondrial Eve and Y Chromosome Adam

Note: This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we locate “mitochondrial eve” and “Y chromosome adam” in their paleontological context.

In the last post in this series, we had arrived, at last, at the origins of our very own species, Homo sapiens. Based on the oldest-known remains in the fossil record, we know that anatomically modern humans were present in Africa at about 200,000 years ago. From this starting point, our species was poised to expand its range into Asia and Europe, starting around 100 KYA (i.e 100 kiloyears ago, a shorthand for 100,000 years ago), and in significant measure from about 50 KYA. In doing so, we would follow, and later encounter, other Homo species that had left Africa before us. Such groups include the Neanderthals and Denisovans, as well as Homo erectus, which, as we have seen, also left Africa and was distributed widely in Asia, including the populations in Indonesia that would form the basis for Eugene Dubois’ seminal discoveries. Neanderthals and Denisovans share a common ancestral population at about 400 KYA, though it is not yet clear if their common ancestral population left Africa or if their lineages separated in Africa and both groups migrated out independently. As for Homo erectus, fossil remains establish that it was widely distributed in Asia as far back as 1.8 million years ago.

Phylogeny diagram
Hominin relationships and approximate divergence times for lineages leading to Neanderthals, Denisovans and modern humans.

It is around this time that we reach a point in human evolution that many evangelicals have at least heard about – the last common female ancestor of all modern humans, popularly known as “mitochondrial Eve”, and the male equivalent – our last common male ancestor, popularly known as “Y-chromosome Adam”.

Mitochondrial Eve and Y-chromosome Adam: common ancestors, but not unique ancestors

Wait just a second, you say – isn’t the evidence strong that modern humans descend from a population that has never numbered less than about 10,000 individuals (and as such, is a topic of significant theological consideration)? How is it, then, that all humans can share a single woman and single man as common ancestors? The short answer is that all humans do share a single man and single woman as common ancestors – but that these ancestors are not our unique, or sole, ancestors. Rather, they both come from that population of about 10,000 individuals – the evidence for which (and the theological questions it raises) we will discuss in upcoming posts.

Understanding how humans can have single maternal and paternal ancestors within a genetically diverse population requires us to take a brief excursion into genetics, and specifically how certain forms of DNA are inherited. As we have discussed previously, our mitochondria have their own small chromosome as a remnant of their time as free-living bacteria. In humans, mitochondria are passed down only from mother to child: sperm do not donate mitochondria to the fertilized egg. As a result, mitochondrial DNA is inherited through the maternal lineage only, in contrast to regular chromosomal DNA, which is inherited through both maternal and paternal lineages. The maternal-specific pattern of inheritance for mitochondrial DNA lends itself to certain mitochondrial variants “taking over” a population, which we can illustrate using a large family tree, or pedigree. (A note about pedigree symbols: circles represent females; squares represent males; a horizontal bar connecting them represents a mating; and a vertical bar from a mating is connected to the offspring of that mating.)

In the pedigree below, we see a large extended family that shows the inheritance of three mitochondrial variants (labeled with different colors). In order to keep the pedigree compact enough to show, the dashed lines indicate matings that connect to one another by wrapping around from one side to the other. As we can see, the red “Mito 3” variant has taken over, or “swept” this population. All individuals in the most recent generations of this family share the woman at the top right as a common ancestor for their mitochondria:

Pedigree diagram

Using the same pedigree, let’s now trace some hypothetical Y-chromosome variants. Y chromosomes, obviously, are passed only from father to son, giving a paternal-specific pattern of inheritance. This pattern, like the maternal-specific pattern for mitochondrial inheritance, can also lead to certain variants easily sweeping a population. Let’s suppose that this same family also has three Y-chromosome variants in the oldest generations:

Pedigree diagram

In this case, the “Y chromosome 1” variant sweeps the population, and everyone in the most recent generations has the man highlighted in yellow as their most recent, common male ancestor.

Now that we have identified a common female and male ancestor of the most recent generations in this pedigree, we can illustrate that they are not their unique ancestors. Both the mitochondrial “eve” and Y-chromosome “adam” of this family come from a larger population – and we can easily show this by looking at variation present on regular chromosomal DNA – the kind passed down through both maternal and paternal lineages.

Let’s return to the exact same pedigree, but now illustrate variation on regular chromosomal DNA with different colors. It is now much harder for this variation to sweep a population in short order, because this variation can be passed on by both males and females:

Pedigree diagram

In contrast to the patterns for mitochondrial and Y-chromosome DNA, we see a diversity of regular chromosomal DNA variation transmitted from the oldest generations down to the most recent ones. For example, consider the middle couple in the first generation. While their mitochondrial and Y-chromosome variation has been lost from this population, the regular chromosomal variation of the male (represented as the blue line) has come down to the present day without trouble. As such, we have a “record” of his ancestry in the population, even after his Y-chromosome variation was lost. Similarly, consider the female in the left couple in the first generation. Though her mitochondrial variation was lost, her regular chromosomal variation (represented as the red line) has been passed down. The total amount of genetic variation on regular chromosomes thus is a tool for determining how many ancestors this population has.

It is this variation in regular chromosomal DNA that indicates that this population has not undergone a drastic reduction in population size in the recent past – and that, even though we can point to recent common ancestors for mitochondrial DNA and Y chromosomes, these common ancestors came from a genetically diverse population. So too with our own lineage – we too have a common maternal ancestor of our mitochondrial DNA (“mitochondrial Eve”), as well as a common paternal ancestor for Y-chromosome DNA (“Y-chromosome Adam”). The diversity of our regular chromosomal DNA, however, shows us that these individuals were part of a large, genetically-diverse population. As in the example we’ve worked with, we know this because of the diversity of regular chromosomal DNA we see in modern human populations.

So, why the excitement over these two individuals? In many ways it’s overblown. These individuals are notable solely for being the last common ancestors of only one small part of our genomes (mitochondrial and Y-chromosome DNA, respectively). While an interesting fact, they were not noticeably different from others in their respective populations. If scientists had not labeled them with names alluding to the biblical narrative, they would likely be little-known among Christians.

Locating Mitochondrial Eve and Y-chromosome Adam in time

Current estimates place mitochondrial Eve just after the dawn of Homo sapiens as recorded in the fossil record, at about 180 KYA. This places her within our species. Until recently, Y-chromosome Adam was dated later, at about 50 KYA, the time of significant human migration out of Africa. Recently, however, a rare Y-chromosome variant has been found in modern humans that pushes back the last common ancestor of all human Y-chromosome DNA to approximately 210 KYA - which, interestingly enough, is right at the cusp of our own species as recorded in the fossil record. Since our species arose as a continuous population that gradually diverged from other hominins, there is no reason to expect that all of our DNA variation will come back to a common ancestor (or coalesce, to use the technical term) within our species. Indeed, some of our regular chromosomal variation does not coalesce within our species or even as far back as our common ancestral population with chimpanzees. As we have discussed before, “species” is a term of convenience that biologists use to attempt to draw a line on what is in fact a gradient of gradual change – and biologically, our species is no exception.

In the next post in this series, we will further explore the fuzzy boundaries of our own species as we travel with some of our ancestors out of Africa – and encounter other hominin species in the process.

For further reading


Dennis Venema is professor of biology at Trinity Western University in Langley, British Columbia. He holds a B.Sc. (with Honors) from the University of British Columbia (1996), and received his Ph.D. from the University of British Columbia in 2003. His research is focused on the genetics of pattern formation and signaling using the common fruit fly Drosophila melanogaster as a model organism. Dennis is a gifted thinker and writer on matters of science and faith, but also an award-winning biology teacher—he won the 2008 College Biology Teaching Award from the National Association of Biology Teachers. He and his family enjoy numerous outdoor activities that the Canadian Pacific coast region has to offer. Dennis writes regularly for the BioLogos Forum about the biological evidence for evolution.

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PNG - #84758

March 13th 2014

When mito Eve and Y-Adam first made the news, it was just some weird thing the scientists were up to. However, it the last decade or so, a fair number of laymen have jumped into the related science by way of genetic genealogy. Genetic genealogy is an avocation which has only emerged since the turn of this century, as companies formed to test the DNA of private individuals towards the goal of tracing ancestors in the genealogical time frame (about the last 8 centuries.) Initially, testing focused on Y chromosome markers because they are inherited like surnames, but attention has now expanded to include mitochondrial haplotypes for the matrilineal line and autosomal and X testing for finding relatives over all one’s ancestral lines over the last few generations. Offerings by a National Geographic and a few companies have extended the testing to deep ancestry going back millennia by testing Y, mitochondrial and autosomal single nucleotide variants.

It’s fascinating to be able to be able to find 8th cousins this way, and trace yourself to some unknown Bronze Age proto-German, as I have. It started me thinking that evangelicals are probably as much into genealogy as the rest of the population is. The ones who indulge in DNA testing are going to have to learn a little genetics to understand their results, and some are bound to see that there is a continuum of inference leading from family finding back to  population migrations, including the migration out of Africa, and ultimately to other hominins and our common ancestor with chimps. I wonder if this will lead to at least some evangelicals realizing that the results of genetics can’t be ignored or denied indefinitely.


Hanan D - #84762

March 13th 2014

As we have discussed before, “species” is a term of convenience that biologists use to attempt to draw a line on what is in fact a gradient of gradual change – and biologically, our species is no exception.

 

So now I will copy and paste the same question I had for you last time:

 

“With gradualism, how does one ever confidentally say NOW we have humans? I am assuming you know where this leads to right? How does the image of God and soul enter the conversation when everything is about small incremental changes?”


Eric Hansmeier - #84769

March 14th 2014

Is soul something humans uniquely possess? I have my doubts. Is the image of God a quality of humanness, or does it reflect the role God has assigned to humans? If the latter, does the assignment have to have been made at a moment in time, or was it also part of the gradual process by which all of life evolved? Perhaps the image of God is itself a result of the process of non-random natural selection.


Merv - #84802

March 17th 2014

With gradualism, how does one ever confidentally say NOW we have humans?

Maybe the same way somebody can assert with 100% certainty:  “I am now a responsible adult human” even though there is no moment in time anybody can point to where that transition from childhood occurs.  The lack of sudden transition in no way negates the full reality of the category.


Jon Garvey - #84808

March 18th 2014

Merv

There is some difference between the two cases, surely? The responsible adult is actually, in a sense, present in the child and even the embryo - the transition is an “evolution” in the etymological sense of an unfolding of what is inherent. Given the usual processes of growth, learning, imitation etc it would be an aberration if an adult didn’t result. “The child is father of the man.” So if our category is “man”, the child is included within it even though full manhood is not yet expressed.

The conceptual difference with evolution is obvious: an organism is nothing except what it is, and its potential is said to be multiple, depending on what direction undirected variation and the environment give its descendants.

We can perhaps say, “I am human” now - but what does that mean? I imagine some highly domesticated dogs might labour under the illusion they’re human. And some mad people think they’re dogs. Is “humanity” some set of characteristics common to all “humans” (in which case it might exclude certain hominins, or even some disabled or undeveloped people arbitrarily), or is humanity a category that determines what characteristics we usually have?

I’m not sure it’s possible to define humanity scientifically - yet that definition is important, as historically recent genocide, slavery or eugenics programs show. Is this not a case where science must give way to religion?


Merv - #84810

March 18th 2014

Differences are there to be sure, but at the general conceptual level I’m critically questioning (that gradualism must always be seen as a threat to any ontological category) I am just offering a simple counterexample where there would be little disagreement –many more can be offered …  the atmosphere is categorically 100.0% real despite the fact that it has no real upper boundary.

Wouldn’t you agree that the statement “I am human now” can still have its full theological meaning even if we find it impossible to identify some exact cut between “pre-humans” and “full humans”?  And you bring up great examples showing that our scenario is far from hypothetical.  The law has to draw a line somewhere to know whether an action should be considered outright murder / abortion / or mere contraception.  The impossibility of science to answer this legal/ethical question definitively is testimony to how right you are that science must give way to other religious/metaphysical/philosophical/legal wisdoms.  We are agreed.  But can we also agree that the difficulty (even among Christians) of agreeing simply that a human life begins at conception – that any alleged “fuzziness” of definition there does not at all threaten our valid category of “full humanity”?

I’m only challenging the simple application of gradualism as some kind of universal solvent for any form of realism.  I grant that I may just not be philosophically educated enough to appreciate real difficulties that may arise from gradualistic understandings.  But at least on a simplistic level they must be somewhat reconcilable or else the whole enterprise of evolutionary science (which inescapably must involve gradualism, punctuated equilibrium theories notwithstanding) among theists is doomed from the outset.  And while some may be happy to “concede” exactly that, I take it Jon that you are not one of them.  Perhaps Hanan is.  But I would be eager to hear more of how you reconcile theistic evolutionary theory such as what you do accept with any gradualisms that you also accept as validly scientific outlooks.

It’s fine to note that “The conceptual difference with evolution is obvious: an organism is nothing except what it is, and its potential is said to be multiple…”  But it may be begging the question just a bit as it may not be obvious to somebody else why an organism must be potentially “multiple” while the same description is denied an embryo except in degree of comparison.  You use the phrase “unguided variation”—which, if we truly accepted that, and I know you don’t, would indeed leave everything open ended.  But as (non-open) theists here who have no truck with “unguided” as a modifier for anything, can’t we speculate that the early organism’s evolutionary potential is as latent in it as an adult’s form is in the embryo?

Thanks for your patience, Jon, in perhaps having to repeat many things you’ve already written extensively about over at the hump.  Some of it does sink in, but sometimes repetition is necessary, at least for me if not others.


Jon Garvey - #84814

March 18th 2014

A quickie Merv (have to rush out soon).

Yup - gradualism does not necessarily destroy categories, but categories tend to challenge gradualism! I guess my main aim is to get people thinking that, in a gradualist scenario, our use of categories like “human” must either be arbitrary (eg we can say who’s human now and forget any transitional forms in the past), or bring God into the picture as the One who decides where the categories are. The latter ought to have some effect on our ways of thinking aboiut biology, I think - the Christian needs to be asking, “What is man?”

Of course, thinking of a teleological form of evolution, what you say about the comparison with human development is quite true: in God’s eyes an Australopithecine could be an unfolding stage of man’s arrival.

But if one brings contingency into the picture that’s less obviously so: in a materialistic evolution, the same Australopithecine is just a lump of clay for chance to mould, and could revert to a less “developed” form (in intelligence or toolmaking, say) or even become extinct, and man never appear.


GJDS - #84826

March 19th 2014

“....why an organism must be potentially “multiple” while the same description is denied an embryo except in degree of comparison.”

So many questions, so much science, and so little understanding. I often ask (with no answer from anyone) how can we have an human embryo without a mother? How does evolutionary thinking get us to discuss these as seperate ‘catagories’? There is nothing latent about a child conceived within a woman’s womb ... nor the fact that humanity is found there. It is a tragic testimony to the Sciences and postmodernity that these self evident truths are now areas for debate and doubt.


Jon Garvey - #84827

March 19th 2014

GJDS

You’re  not saying that “pre-implantation zygotic material” is human!

I know that’s what I was taught consistently in embryology at Cambridge (when it was called “a human zygote”), but that was before people wanted to harvest embryonic stem cells and so on.


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